TW-EDM method and apparatus with a ferromagnetic wire electrode

ABSTRACT

A method of and an apparatus for machining an electrically conductive workpiece with a wire electrode which is originally ferromagnetic or becomes ferromagnetic in the machining zone wherein the magnetic properties of the wire are detected as it passes out of the machining zone and the detected magnetic properties are used to control a machining parameter.

FIELD OF THE INVENTION

The present invention relates to traveling-wire electrical dischargemachining (TW-EDM), or a process of electrical-discharge machining of aconductive workpiece by means of a thin, elongate continuous electrodeelement (hereinafter referred to as "wire electrode" or "wire"), thewire being axially advanced from supply means to travel through amachining zone in the workpiece and to be taken up onto takeup means, inwhich process a succession of time-spaced electrical discharges areeffected between the traveling wire and the workpiece across a machininggap in the presence of a flushing liquid medium to electroerosivelyremove material from the workpiece while the wire and the workpiece arerelatively displaced transverse to the wire along a programmed path toadvance erosive material removal through the workpiece and thereby toform a contour corresponding thereto in the workpiece.

In particular, the invention relates to the TW-EDM process of the typein which the traveling wire is ferromagnetic at least downstream of thecutting zone in its path of axial travel, and to an improved method ofand apparatus for performing the process described whereby changingprocess conditions in the machining zone are ascertained by monitoring aproperty of the wire advancing out of the machining zone towards thetakeup means to provide signals which can be used and adaptively controlmachining parameters to eliminate a possibly damaging condition for thethin wire and maintain the machining process with a high efficiency.

BACKGROUND OF THE INVENTION

In the TW-EDM art, attempts to obtain higher removal rate with duemachining quality and economy, give rise to various problems vis a visthe wire electrode. The wire must be capable of carrying electricaldischarge current of high amplitudes to achieve high rates of erosionand yet must be relatively thin (typically 0.05 to 0.5 mm in thickness)to assure high cutting accuracy. To allow the wire and hence the erosionto faithfully follow the programmed machining path, a high tension mustbe applied to the wire to maintain its active electrode length travelingthrough the cutting zone as straight or geometrically accurate aspossible. Since the wire is consumed or "used up", it is economicallydesirable that it be "used up", that is, advanced to renew the activeelectrode surface through the cutting zone, as slowly as possible. Sincethe wire is thin, the cutting groove created behind the traversing wirein the workpiece is correspondingly narrow so that machining dischargeproducts tend to accumulate in the cutting zone through which the wiretravels. This tendency increases as the thickness of the workpiece orthe active electrode length of the wire increases. All these factors andrequirements severely affect the ability of the wire to withstandbreakage. In TW-EDM, wire breakage is fatal since it interrupts themachining process. Therefore, often higher removal rates cannot bepursued without overcoming the higher risk of wire breakage which arisesfrom the use of greater energy and higher repetition rate of machiningdischarge pulses are required to increase the erosion rate.

In TW-EDM, machining is considered to proceed in a normal mode whenelectrical discharges occur uniformly throughout the active electrodesurface of the wire traveling through the cutting zone. Under anadequate set of operating parameters, the wire will not then break ifmachining is allowed to proceed at a maximum efficiency by permittingpulses applied to result in such random electrical discharges at ahighest proportion and/or the wire to travel at a minimum speed. Thewire passed out of the cutting zone can then be considered to have beentruly "used up" upon removing stock from the workpiece at maximumefficiency. As long as machining proceeds in the normal mode, the pulsesand the contouring feed can be enhanced to increase removal rate to amaximum level which the eventual set of operating parameters allows.

In a TW-EDM operation, however, it is practically not possible toexclude the possibility that such a normal machining mode is disturbed.Machining is more likely disturbed as higher removal is attempted duringan "enhanced" machining condition whereas machining conditions arerelatively "low" or safer against wire breakage at relatively lowremoval rates; in the latter case machining if disturbed is more likelyreturned spontaneously to the normal mode. Machining is considered tobecome disturbed if a uniform distribution of successive electricaldischarges over the entire active electrode surface of the wiretraveling through the cutting zone is impaired. If this conditioncontinues or is allowed to continue, discharges tend to become"abnormal" in the cutting zone. The random electrical discharges willprogressively be reduced in number and concentrate at particular sitesin the cutting zone. Eventually the wire will be broken at a certainpoint where its strength is the weakest or it can no longer withstandexcessive thermal and mechanical stresses due to the localized abnormalelectrical discharges.

While it has thus been generally recognized that wire breakage is ofmajor concern and is in effect triggered by a disturbance of the normalmachining mode and an abnormal concentration of electrical discharges,difficulties have been encountered in the prior art in timely detectinga real tendency for the wire to break or properly predicting wirebreakage during the machining process. It has been found that electricaldetection of discharges themselves most often does not provide adequate,valid and reliable information to this end. Accordingly, given aparticular electrode wire, usually one is compelled to choose safermachining conditions with respect to the possibility of its breakage, ormust be satisfied with a removal rate which is much lower than thatwhich ought to be readily obtainable if the wire does not break.

OBJECTS OF THE INVENTION

It is, accordingly, an important object of the present invention toprovide a valid, reliable and efficient way of and means for detecting areal tendency toward breakage of the traveling wire electrode and, moregenerally, varying process conditions in the TW-EDM machining zone.

Another important object of the invention is to provide a new andimproved method of and apparatus for electrical-discharge-machining anelectrically conductive workpiece with a traveling wire electrodewhereby machining is allowed to proceed with maximum efficiency.

A further object of the invention is to provide a novel and improvedtraveling electrode wire which has greater ability to withstand breakagethan and to afford greater removal rate than that obtainable with, theconventional electrode wires in the TW-EDM process.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided, in a firstaspect thereof, a method of machining an electrically conductiveworkpiece in which a thin, elongate continuous electrode is axiallytransported from a supply means to travel through a machining zone inthe workpiece and to be taken up onto a take-up means and a successionof electrical discharges are effected between the traveling electrodeand the workpiece across a machining gap in the presence of a flushingliquid medium to electroerosively remove material from the workpiecewhile the traveling electrode and the workpiece are relatively displacedtransversely to the traveling electrode along a programmed path toadvance erosive material removal therealong and in which the electrodeis ferromagnetic at least downstream of the cutting zone in its path oftravel, which method comprises sensing a magnetic property of thetraveling electrode downstream of the machining zone in the path oftravel to produce an electrical signal representing a disturbance of theelectrical discharges from a normal mode in the machining zone. The saidelectrical signal is advantageously employed to control at least onemachining parameter affecting a distribution of the electricaldischarges so as to restore the normal mode in the machining zone.

The invention provides, in a second aspect thereof, an apparatus formachining an electrically conductive workpiece with a thin, elongatecontinuous electrode, which apparatus comprises means for axiallytransporting the electrode from supply means along a predetermined pathof travel to cause it to travel through a machining zone in theworkpiece and to be taken up onto takeup means; means for supplying aflushing liquid medium into a machining gap formed between the travelingelectrode and the workpiece; power supply means for effecting asuccession of electrical discharges across said machining gap betweenthe workpiece and the electrode traveling through the machining zone toelectroerosively remove material from the workpiece; drive means forrelatively displacing the traveling electrode and the workpiecetransversely to each other along a programmed path to advance erosivematerial removal therealong; and means disposed downstream of saidcutting zone in said predetermined path of travel for sensing a magneticproperty of the wire passed from the cutting zone to produce a signalrepresenting a disturbance of the electrical discharges from apredetermined normal machining mode. Means is advantageously providedresponsive to said electrical signal for controlling at least one ofmachining parameter affecting a distribution of the electricaldischarges so as to restore said normal machining mode in the machiningzone.

The invention also provides, in a third aspect thereof, a thin, elongatecontinuous electrode element for TW-EDM, which element is composed of amaterial which consists of 0.05 to 3.0% by weight of at least onerare-earth element and the balance a steel material. The steel materialpreferably consists of 0.29 to 0.66% by weight carbon, 0.15 to 0.35% byweight silicon, 0.3 to 0.6% by weight manganese, not greater than 0.04%by weight phosphorus, not greater than 0.04% by weight sulfur and thebalance iron. Preferably, the electrode is constituted of a core wirecomposed of a steel material of the composition described and having alayer of a more electrically conductive substance such as copper orbrass.

BRIEF DESCRIPTION OF THE DRAWING

These and other features of the present invention as well as advantagesthereof will become more readily apparent from the following descriptionwhen taken with reference to the accompanying drawing in which:

FIG. 1 is a schematic view diagrammatically illustrating a TW-EDM systemaccording to the present invention;

FIG. 2 is a schematic circuit diagram of the magnetic detector used inthe system of FIG. 1;

FIG. 3 is a schematic circuit diagram of the magnetizing unit which maybe used in the system of FIG. 1;

FIG. 4 is a cross-sectional view diagrammatically illustrating acomposite wire electrode for use in the system of FIG. 1, comprising acore wire of a steel material having a layer of an electricallyconductive metal or alloy coated thereon;

FIG. 5 is a similar view diagrammatically illustrating another compositewire electrode for use in the system of FIG. 1, comprising a core wireof an electrically conductive metal or alloy and having a layer of aferromagnetic material coated thereon; and

FIG. 6 is a schematic circuit diagram of a demagnetizing unit which maybe used in the system of FIG. 1.

SPECIFIC DESCRIPTION

FIG. 1 shows a TW-EDM apparatus incorporating the present invention. Anelectrode wire 1, typically of a thickness of 0.05 to 0.5 mm, dispensedfrom supply means in the form of a supply reel 2 is guided to pass overa guide roller 3, between abutting rollers 4 and 5, over a guide roller6, through a workpiece 7, over a guide roller 8, between abuttingrollers 9 and 10 and over a roller 11 for takeup onto a takeup meansincluding a takeup reel 12. The rollers 9 and 10 are driven by means ofa motor (not shown) to axially transport the wire 1 from the supply reel2 onto the takeup reel 12 through a cutting zone in the workpiece 7between a pair of wire-positioning guide members 13 and 14. The rollers4 and 5 may also be driven and serve as a brake to give a sufficienttension on the traveling wire 1 so that the wire 1 provides a straighttraveling cutting electrode length in electroerosive machiningrelationship with the workpiece 7 across a machining gap between thepositioning guide members 13 and 14. The machining gap is supplied witha flushing liquid medium, e.g. deionized water, from one or more nozzles15 which communicates with a supply 28 of the pressurized flushingmedium. The wire 1 is electrically connected via one or more conductingpins 16 to one terminal of an EDM power supply 17 whose other terminalis electrically connected via an electrically conductive clamp 18 to theworkpiece 7. The EDM power supply 17 provides a succession of electricalvoltage pulses which may result correspondingly in electrical dischargespassed between the traveling wire 1 and the workpiece across the gap toelectroerosively remove stock from the workpiece 7. The clamp 18 servesto secure the workpiece 7 to a stand 19 securely mounted on a worktable20 which is in turn movably mounted on a base 21 of the EDM apparatus.The worktable 20 is driven by a pair of motors Mx, My under commands ofa numerical control (NC) unit 29 to displace the workpiece 7 relative tothe traveling wire 1 along a programmed path of feed in a horizontalplane to advance the erosive stock removal along that path, therebyforming a corresponding contour in the workpiece 7. The elements 2-5 and9-12 are shown as mounted on a column 22 standing upright on the base21. The elements 6, 13 and 16 are mounted on a tool head 23 which iscarried on the end of an upper horizontal arm 24 which in turn extendsfrom the column 22 and the elements 8 and 14 are mounted on the end of alower horizontal arm 25 also extending from the column 22.Electrode-vibrating units 26 and 27, which may be of the type describedin U.S. Pat. No. 4,321,450 and U.S. Pat. No. 4,358,655, are also shownto be associated with the wire-positioning guide members 13 and 14,respectively, for imparting high-frequency mechanical vibrations to thewire 1 traveling through the cutting zone.

In accordance with the present invention a magnetic detector 30 isprovided downstream of the machining zone in the path of travel of thewire 1 for sensing a magnetic property of the wire 1 passed out of themachining zone, and is here shown as disposed between the guide roller 8and the drive rollers 9, 10. By monitoring a magnetic property of thewire which has passed out of the machining zone, it has been found thata disturbance of the normal mode in the machining zone can be accuratelydetected.

In one embodiment of the invention, it should be noted that aconventional non-magnetic or paramagnetic electrode wire such as acopper or base wire may be employed if the workpiece 7 is aferromagnetic material such as a magnetic steel or a metal carbidecemented with cobalt. It has been found that when an eroding electricaldischarge is effected between a point of the wire electrode and theworkpiece, thus removing material from the latter, an amount of theremoved workpiece material tends to deposit on that point of the wire.Thus, a uniform distribution of electrical discharges over the cuttingzone results in a deposit of the magnetic material uniformly distributedon the active wire electrode surface. Accordingly, a disturbance of thedesired uniformity of the magnetic property over a length of the wireoutgoing from the machining zone indicates a disturbance of the normalmode of electrical discharges in the machining zone.

As shown in FIG. 2, the magnetic detector 30 may comprise an annularcoil 31 constructed and arranged coaxially with the wire 1 in a magneticsensing relationship therewith. The detector 30 may also take a magneticpick-up having a sensing coil and disposed in a magnetic sensingrelationship with the wire 1. The output of the sensing coil 31 is fedvia an amplifier 32 to a detecting circuit 33 whose output is in turnconnected to a counter 34. As the wire 1 is displaced through thesensing coil 31, the detecting circuit 33 produces an output pulse eachtime the sensing coil 31 fails to detect magnetism during each incrementof the displacement of the wire 1. Thus, if the sensing coil 31continues to fail to detect magnetism, pulses are consecutively producedfrom the detecting circuit 33. These pulses are connected by the counter34 and when the number of pulses counted reaches a level preset therein,indicating a disturbance of the electrical discharges from the normalmode in the machining zone, the counter 34 produces a control signal.The control signal may be applied to the EDM power supply 17 to modifyone or more of the discharge parameters such as the peak current Ip, theduration τon and the pulse interval τoff of the discharge pulses, to thefluid supply 15a to increase the pressure and/or flow rate of theflushing medium into the machining zone, to the NC unit 29 to reduce therate of machining feed, and/or to the electrode vibration units 26, 27to initiate imparting high-frequency vibrations to the wire 1 travelingacross the workpiece 7.

To facilitate detection of magnetism by the magnetic detector 30, thearrangement of FIG. 1 includes an auxiliary unit 35 which is disposedimmediately upstream of the magnetic detector 30 or between the cuttingzone and the magnetic detector 30 in the path of wire travel. In theembodiment being described, the auxiliary unit 35 is constituted by amagnetizing unit 40 as shown in FIG. 3. The magnetizing unit 40 showncomprises a magnetizing coil 41 surrounding the wire 1, a capacitor 42connected with the coil 41 via a switch 43, and a DC source 44 connectedwith the capacitor 42 via charging impedances 45. The capacitor 42 isalternately charged from the DC source 44 and discharged through thecoil 41 as the switch 43 is periodically turned on and off by anoscillator 46 to develop a succession of electrical pulses in the coil41 and a corresponding pulsed magnetic field therethrough so that adeposit of the magnetic material from the workpiece 7 if present on thewire traveling through the coil 41 may be instantaneously magnetized toa full magnetization level.

Another embodiment of the invention makes use of the wire 1 which isalready ferromagnetic prior to entry into the machining zone. Thus, thewire may be composed homogeneously of a ferrous material or steel ormay, as shown in FIG. 4, be constituted by a steel or ferrous core wire1a coated with a layer 1b of metal or alloy of a higher electricallyconductivity such as copper, brass or aluminum. Alternately, the wire 1may, as shown in FIG. 5, be composed of a core wire 1c of such a metalor alloy coated with a layer 1d of a magnetic material such as a ferriteor a mixture of the ferrite with the metal or alloy.

The steel or ferrous material is preferably composed of a hard or pianowire steel, e.g. JIS3506-SWRH62A (Japanese Industrial Standard) orJIS3506-SWRH32A (Japanese Industrial Standard) but may also be of ahigh-carbon steel, a magnetic stainless steel, a chromium orchromium-molybdenum steel, an iron-manganese-titanium alloy or aniron-chromium-cobalt alloy. Since such a material is tough, the wire 1can be of especially reduced in thickness. It has been found that wire1, which has a strength far greater to withstand breakage and affordshigher removal rate and machining stability, than the conventionalTW-EDM wires, can be provided when the wire 1 or the core wire 1a iscomposed of a material containing 0.05 to 3% by weight one or morerare-earth elements such as yttrium, samarium, lanthanum or cerium or inthe form of misch metal, and the balance a hard or piano steel,preferably the steel which consists by weight of 0.59 to 0.66% carbon,0.15 to 0.35% silicon, 0.3 to 0.6% manganese, not greater than 0.04%phosphorus, not greater than 0.04% sulfur and the balance iron, or thesteel which consists by weight of 0.29 to 0.36% carbon, 0.15 to 0.35%silicon, 0.3 to 0.6% manganese, not greater than 0.04% phosphorus, notgreater than 0.04% sulfur and the balance iron.

When such a ferrous wire 1 with a uniform magnetic property over itslength is passed through the cutting zone, an electrical discharge whileremoving stock from the workpiece 7 will also effect a localizedheat-treatment of each point on the wire at which it strikes and hencesignificantly alter the magnetic property of that point. This effect isfurther enhanced by an instantaneous magnetic field induced by thedischarge current in the vicinity of the discharge site. An alternationof the magnetic property will result uniformly over the entire electrodesurface of the wire passed out of the cutting zone if the electricaldischarges are taking place uniformly over the entire cutting zone. Ifthe electrical discharges are concentrating at one or more points orsites on the wire 1 passing through the cutting zone, this is indicatedby a length of the wire out of the cutting zone in which length themagnetic property has not materially altered. Thus, the magneticdetector 30 disposed downstream of the cutting zone is capable ofproviding at the output of the counter 34 a signal representing adisturbance of electrical discharges from the normal mode in the cuttingzone. The alternation of the magnetic property at a discharge-strickenzone of the wire is observed by the fact that each such zone ismagnetically hardened. The wire 1 with such magnetically hardened zonespasses through the magnetizing unit 35 and then into the magneticdetector 30 in the arrangement of FIG. 1. Thus, the magneticallyhardened zones are magnetized to a full magnetization level to develop agreater magnetic flux to facilitate sensing by the magnetic pickup orthe sensing coil 31.

The arrangement of FIG. 1 is also shown to include a further orauxiliary assembly 36 disposed upstream of the cutting zone and whichmay include a demagnetizing unit 50. As shown in FIG. 6, thedemagnetizing unit 50 comprises a coil 51 surrounding the ferromagneticwire 1 traveling into the cutting zone and a current supply for passinga damping AC through the coil 51. The demagnetizing unit 50 is providedto eliminate any magnetization which the wire 1 may have had as suppliedwith or stored on the reel 2 or as traveling through the supply side ofthe system in its path of travel, and thus to ensure that the wire 1traveling into the cutting zone is uniform in the magnetic property overits length.

The auxiliary magnetic assembly 36 may also include a magnetizing unit40 as shown in and previously described in connection with, FIG. 3. Themagnetizing unit 40 in this case is designed to regularly magnetize theferromagnetic wire 1 traveling into the cutting zone. A series of smallmagnetic units, each with N and S poles, may be produced in theferromagnetic wire 1 along its length. The regular distribution of thesemagnetic units tends to be altered as the wire 1 is passed through thecutting zone while being subjected to eroding electrical discharges. Ifa disturbance of the electrical discharges is created in the cuttingzone, there results a length of the wire 1 in which the originaldistribution of the small magnetic units remains substantiallyunchanged. The magnetic detector 30 responds to the occurrence of such alength and provides a control signal which may be used to control one ormore machining parameters so as to restore the normal mode in themachining zone, as previously described.

What is claimed is:
 1. A method of machining an electrically conductiveworkpiece in which a thin, elongate continuous electrode is axiallytransported from supply means to travel through a machining zone in theworkpiece and to be collected into takeup means and a succession ofelectrical discharges are effected between the traveling electrode andthe workpiece across a machining gap in the presence of a flushingliquid medium to electroerosively remove material from the workpiecewhile the traveling electrode and the workpiece are relatively displacedtransversely to the electrode along a programmed path to advance erosivematerial removal therealong and in which the electrode is ferromagneticat least downstream of the machining zone in its path of travel, themethod comprising the step of:sensing a magnetic property of saidtraveling electrode downstream of said machining zone to produce anelectrical signal representing a disturbance of said electricaldischarges from a normal mode in said machining zone.
 2. The methoddefined in claim 1, further comprising the step of controlling, inresponse to said electrical signal, at least one machining parameteraffecting a distribution of the electrical discharges so as to restoresaid normal mode in said machining zone.
 3. The method defined in claim1 or claim 2 wherein said workpiece is composed of a ferromagneticmaterial and said electrode is essentially paramagnetic upstream of saidcutting zone, further comprising the step of rendering said electrodeferromagnetic through said machining zone by forming a deposit offerromagnetic material in a region of a said electrical discharge onsaid electrode in said machining zone.
 4. The method defined in claim 3,further comprising the step of, prior to sensing said magnetic property,magnetizing said electrode passing out of said machining zone.
 5. Themethod defined in claim 1 or claim 2 wherein said electrode is composedat least in part of ferromagnetic material.
 6. The method defined inclaim 5 wherein said electrode comprises a core element composed of saidferromagnetic material and having a layer of metal or alloy coatedthereon which is higher in electrical conductivity than saidferromagnetic material.
 7. The method defined in claim 5 wherein saidelectrode comprises a core element composed of metal or alloy which isessentially paramagnetic and having a coating thereon which is at leastin part composed of said ferromagnetic material.
 8. The method definedin claim 5 wherein said ferromagnetic material consists of 0.05 to 3% byweight of at least one rare-earth element and the balance a steelmaterial.
 9. The method defined in claim 5, further comprising the stepof, prior to sensing said magnetic property, magnetizing said electrodepassing out of said machining zone.
 10. The method defined in claim 5,further comprising the step of demagnetizing said ferromagneticelectrode prior to entry into said machining zone.
 11. The methoddefined in claim 10, further comprising the step of magnetizing saiddemagnetized ferromagnetic electrode prior to entry into said machiningzone.
 12. The method defined in claim 5, further comprising the step ofmagnetizing said ferromagnetic electrode prior to entry into saidmachining zone.
 13. An apparatus for machining an electricallyconductive workpiece with a thin, elongate continuous electrode,comprising:means for axially transporting said electrode from supplymeans along a predetermined path of travel to cause it to travel througha machining zone in the workpiece and to be taken up onto takeup means;means for supplying a flushing liquid medium into a machining gap formedbetween the traveling electrode and the workpiece in said machiningzone; power supply means for effecting a succession of electricaldischarges across said gap between the workpiece and the electrodetraveling through said machining zone to electroerosively removematerial from said workpiece; drive means for relatively displacing saidtraveling electrode and the workpiece transversely to said electrodealong a programmed path to advance erosive material removal therealong;and means disposed downstream of said machining zone in saidpredetermined path of travel for sensing a magnetic property of saidelectrode passing out of said machining zone to produce a signalrepresenting a disturbance of the electrical discharges from apredetermined machining mode in said machining zone.
 14. The apparatusdefined in claim 13, further comprising means responsive to said sensingmeans for controlling at least one machining parameters affecting adistribution of electrical discharges in said machining zone so as torestore said predetermined machining mode therein.
 15. The apparatusdefined in claim 13 or claim 14, further comprising means disposedbetween said machining zone and said sensing means in said predeterminedpath of travel for magnetizing said electrode passing out of saidmachining zone.
 16. The apparatus defined in claim 13 or claim 14,further comprising means disposed upstream of said machining zone insaid predetermined path of travel for demagnetizing said electrode priorto entry into said machining zone.
 17. The apparatus defined in claim 13or claim 14, further comprising means disposed upstream of saidmachining zone in said predetermined path of travel for magnetizing saidelectrode so as to form a predetermined magnetic pattern along itslength prior to entry into said machining zone.